在 Oracle 11.2.0.2 之后，随着一系列 SCN 耗尽问题的出现，很多补丁涌现出来，一个新的 Package 增加进来。

这个 Package 就是 DBMS_SCN。

如果你的数据库中存在这个 Package，也就意味着你已经安装具备了关于 DB Link 的修正补丁。

以下是这个包的主要函数过程以及说明，这个内容来自 Oracle 11.2.0.4 版本平台：

RemRem $Header: rdbms/admin/dbmsscnc.sql /st_rdbms_11.2.0/1 2013/04/18 23:05:40 vgokhale Exp $RemRem dbmsscn.sqlRemRem Copyright (c) 2012, 2013, Oracle and/or its affiliates. Rem All rights reserved. RemRem    NAMERem      dbmsscnc.sql - dbms_scn package definitionRemRem    DESCRIPTIONRem      RemRem    NOTESRem      RemRem    MODIFIED   (MM/DD/YY)Rem    mtiwary     05/26/12 - Declarations and definitions related to DBMS_SCNRem                           package.Rem    mtiwary     05/26/12 - CreatedRemRemRem    BEGIN SQL_FILE_METADATA Rem    SQL_SOURCE_FILE: rdbms/admin/dbmsscn.sql Rem    SQL_SHIPPED_FILE: Rem    SQL_PHASE: Rem    SQL_STARTUP_MODE: NORMAL Rem    SQL_IGNORABLE_ERRORS: NONE Rem    SQL_CALLING_FILE: Rem    END SQL_FILE_METADATASET ECHO ONSET FEEDBACK 1SET NUMWIDTH 10SET LINESIZE 80SET TRIMSPOOL ONSET TAB OFFSET PAGESIZE 100CREATE OR REPLACE LIBRARY DBMS_SCN_LIB TRUSTED AS STATIC;/CREATE OR REPLACE PACKAGE DBMS_SCN AUTHID CURRENT_USER ISDBMS_SCN_API_MAJOR_VERSION  CONSTANT NUMBER := 1; DBMS_SCN_API_MINOR_VERSION  CONSTANT NUMBER := 0;PROCEDURE GetCurrentSCNParams(rsl      OUT number,headroom_in_scn OUT number,headroom_in_sec OUT number,cur_scn_compat OUT number,max_scn_compat OUT number);--      Currently no exceptions are thrown.--      rsl             - Reasonable SCN Limit as of 'now'--      headroom_in_scn - Difference between current SCN and RSL--      headroom_in_sec - number of seconds it would take to reach RSL--                        assuming a constant SCN consumption rate associated--                        with current SCN compatibility level--      cur_scn_compat  - current value of SCN compatibility--      max_scn_compat  - max value of SCN compatibility this database--                        understandsFUNCTION GetSCNParamsByCompat(compat IN number,rsl           OUT number,headroom_in_scn OUT number,headroom_in_sec OUT number) RETURN boolean;--     compat           -- SCN compatibility value--     rsl              -- Reasonable SCN Limit--     headroom_in_scn  -- Difference between current SCN and RSL--     headroom_in_sec  -- number of seconds it would take to reach RSL--                         assuming a constant SCN consumption rate associated--                         with specified database SCN compatibility----     Returns False if 'compat' parameter value is invalid, and OUT parameters--     are not updated.PROCEDURE GetSCNAutoRolloverParams(effective_auto_rollover_ts OUT DATE,target_compat OUT number,is_enabled OUT boolean);--      effective_auto_rollover_ts  - timestamp at which rollover becomes--                                    effective--      target_compat               - SCN compatibility value this database--                                    will move to, as a result of--                                    auto-rollover--      is_enabled                  - TRUE if auto-rollover feature is--                                    currently enabledPROCEDURE EnableAutoRollover;PROCEDURE DisableAutoRollover;END DBMS_SCN;/

这里就可以看到 auto-rollover 的自动 SCN 兼容性终止时间，也就是说，在不同的兼容性设置中，SCN 的算法不同，但是内置了天然的算法过期时间。

在此之后，可以通过命令修改数据库的 SCN 兼容性算法：

SQL> ALTER DATABASE SET SCN COMPATIBILITY 2;

Database altered.

从高级别向低级别修改，需要数据库在 Mount 状态：

SQL> ALTER DATABASE SET SCN COMPATIBILITY 2;ALTER DATABASE SET SCN COMPATIBILITY 2*ERROR at line 1:ORA-01126: database must be mounted in this instance and not open in any instanceSQL> shutdown immediate;Database closed.Database dismounted.ORACLE instance shut down.SQL> startup mount;ORACLE instance started.Total System Global Area 4609830912 bytesFixed Size   2260888 bytesVariable Size   989855848 bytesDatabase Buffers  3607101440 bytesRedo Buffers    10612736 bytesDatabase mounted.SQL> ALTER DATABASE SET SCN COMPATIBILITY 2;Database altered.SQL> alter database open;Database altered.

这是一个非常重要的变化。

原文发布时间为：2018-03-16

本文作者：盖国强

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